The present invention relates to an improved alkaline-earth based sorbent for removing effluent such as sulfur dioxide from a combustion gas stream, a process for preparing this sorbent, and a process for using it.
Processes for removing environmentally harmful effluents from a gaseous combustion stream using sorbents are known in the art. A number of attempts have been made to increase process effectiveness by manufacturing a more effective sorbent for removing these effluents from a gas stream such as combustion flue gases. Some of these attempts have involved the incorporation of iron into a sorbent material. For example, the articles, "Enhanced Sulfur Capture By Promoted Calcium-Based Sorbents", by D. M. Slaughter, S. L. Chen and W. R. Seeker, Proceedings: 1986 Joint Symposium on Dry SO.sub.2 and Simultaneous SO.sub.2 /NO.sub.x Control Technologies (EPRI CS-4966, Volume 1), pages 12-1 to 12-24 (1986) and "The Effectiveness of Additives for Enhancing SO.sub.2 Removal With Calcium Based Sorbents", by L. J. Muzio, G. R. Offen, A. A. Boni and R. Beittel, Proceedings: 1986 Joint Symposium on Dry SO.sub.2 and simultaneous SO.sub.2 /NO.sub.x Control Technologies (EPRI CS-4966, Volume 1), pages 13-1 to 13-23 (1986), both report studies which indicate that the addition of iron to a calcium-based sorbent offers no enhancement in sulfur capture. In both of these studies, the iron source [Fe(NO.sub.3).sub.3 ] was dissolved in water, and the solution mixed with CaO (lime) for preparation of the sorbent. In this formulation, however, as soon as the solution comes into contact with CaO, it becomes alkaline (pH&gt;7). This causes the precipitation of iron hydroxide [FeO(OH)]. The result is that these formulations led to the mixing of a water-insoluble iron compound with a calcium-based sorbent, which is an ineffective method to achieve promotion by iron.
In patent application Ser. No. 657,442, filed Feb. 19, 1991, for "In-Situ Removal of Effluent from a Gaseous Stream by Injection of an Effluent Sorbent Into Downstream of the Combustion Zone", it was disclosed that water soluble salts of iron can enhance the reactivity of alkaline-earth water-insoluble compounds toward sulfur dioxide. In one embodiment described in that application, ferrous gluconate is dissolved in water, and then the solution is mixed with CaO. The resulting material is a dry Ca(OH).sub.2 powder containing the iron promoter. When this material is put in contact at 1100.degree. C. with gases containing sulfur dioxide, up to 60% of the calcium reacts with the sulfur dioxide and oxygen to form CaSO.sub.4 in less than one second. When pure Ca(OH).sub.2 is subjected to the same reaction conditions, only 35% of the calcium can be converted to CaSO.sub.4. This embodiment demonstrates that the effective incorporation of iron can enhance the reactivity of calcium-based materials and therefore, make them better suited sorbents for applications for sulfur dioxide removal.
Sugars by themselves have also been claimed as promoters for calcium-based sorbents used in the removal of sulfur dioxide. College and Vinatry in U.S. Pat. No. 4,626,418, for example, claim that the addition of a sugar solution to CaO during preparation of the sorbent leads to an enhancement in sulfur dioxide removal.
It is an object of the present invention to provide an improved sorbent which may be used to remove effluent from a combustion gas stream.
It is a further object of the present invention to provide an alkaline-earth based sorbent as above which is highly reactive towards sulfur dioxide.
It is still a further object of the present invention to provide a sorbent as above which contains organic compound(s) for ensuring that an iron promoting material remains in solution upon contact with an alkaline-earth based sorbent.
Yet another object of the present invention is to provide a process for making the above sorbent and a process for using it to remove sulfur dioxide from a gas stream.
Still other objects and advantages of the present invention will become clearer from the following description.